An inductive measuring device of the above type provides a galvanic separation of the current to be measured from the measuring circuit which is essential when high currents are concerned.
A drawback of these devices in the case of high currents is the risk that the magnetic circuit used becomes saturated. This can be overcome by providing an air-gap in the magnetic circuit. On the other hand, the presence of an air-gap renders the measuring device sensitive to the influence of parasitic outer magnetic fields.
The invention provides a measuring device for alternating components superposed to a high direct current, such as a power current, which avoids a saturation of the magnetic measuring circuit while reducing to a minimum the influence of parasitic magnetic fields on the result of the measure.
To this effect, the measuring device according to the invention comprises a magnetic measuring circuit which is substantially symmetrical with respect to at least one cross-sectional plane thereof and has two air-gaps of same dimensions formed on either side of said plane of symmetry. This measuring device further comprises two substantially identic measuring coils coupled with the magnetic measuring circuit and an electric measuring circuit coupled to these two coils, the measuring circuit comprising adding means for adding the voltages induced, respectively, into the two measuring coils by the alternating current components and an integrating circuit connected to the adding means for receiving a current representative of the sum of these induced voltages.
The measuring coils of the device according to the invention are preferably connected in series and each of the measuring coils is preferably connected in parallel with a respective voltage divider, at least one of these voltage dividers being adjustable, and the integrating circuit is connected to a series connection of two resistors of same value which are part, respectively, of each of the voltage dividers.
According to a preferred embodiment of the invention, a band-pass filter is connected to the integrating circuit and arranged for filtering the frequencies of the alternating current components to be measured.
According to a preferred embodiment of the invention, the measuring coils are arranged, respectively, inside the air-gaps of the magnetic circuit.
According to another preferred embodiment, the measuring coils are arranged, respectively, around portions of the magnetic circuit between which the air-gaps are formed.
According to an embodiment of the invention, the magnetic circuit can comprise two U-shaped parts of same dimensions, the ends of said parts being arranged opposite each other for forming the air-gaps.
In a preferred embodiment of the invention, screening means are provided which include two screening members of a magnetically permeable material arranged, respectively, at the outside of the magnetic circuit at a determined distance from the air-gaps.
According to a particular embodiment of the invention, the screening members comprise plates or sheets arranged, respectively, in planes substantially parallel to the plane of symmetry of the magnetic circuit, the dimensions of the screening members in these planes being substantially greater than the dimensions of the air-gaps.
According to another preferred embodiment of the invention, the screening members comprise plates or sheets having each a portion arranged in a respective plane parallel to the plane of symmetry of the magnetic circuit and portions bent to partially surround the air-gaps.
According to yet another preferred embodiment, the screening members comprise plates or sheets having each a portion in contact with the magnetic circuit in the vicinity of sections of that circuit lying in the plane of symmetry of the same.